Digital watermarking (sometimes termed “data hiding” or “data embedding”) is a growing field of endeavor, with several different approaches. The present assignee's work is reflected in the patents and applications detailed above, together with laid-open PCT application WO97/43736. Other work is illustrated by U.S. Pat. Nos. 5,734,752, 5,646,997, 5,659,726, 5,664,018, 5,671,277, 5,687,191, 5,687,236, 5,689,587, 5,568,570, 5,572,247, 5,574,962, 5,579,124, 5,581,500, 5,613,004, 5,629,770, 5,461,426, 5,743,631, 5,488,664, 5,530,759, 5,539,735, 4,943,973, 5,337,361, 5,404,160, 5,404,377, 5,315,098, 5,319,735, 5,337,362, 4,972,471, 5,161,210, 5,243,423, 5,091,966, 5,113,437, 4,939,515, 5,374,976, 4,855,827, 4,876,617, 4,939,515, 4,963,998, 4,969,041, and published foreign applications WO 98/02864, EP 822,550, WO 97/39410, WO 96/36163, GB 2,196,167, EP 777,197, EP 736,860, EP 705,025, EP 766,468, EP 782,322, WO 95/20291, WO 96/26494, WO 96/36935, WO 96/42151, WO 97/22206, WO 97/26733. Some of the foregoing patents relate to visible watermarking techniques. Other visible watermarking techniques (e.g. data glyphs) are described in U.S. Pat. Nos. 5,706,364, 5,689,620, 5,684,885, 5,680,223, 5,668,636, 5,640,647, 5,594,809.
Much of the work in data embedding is not in the patent literature but rather is published in technical articles. In addition to the patentees of the foregoing patents, some of the other workers in this field (whose watermark-related writings can by found by an author search in the INSPEC or NEXIS databases, among others) include I. Pitas, Eckhard Koch, Jian Zhao, Norishige Morimoto, Laurence Boney, Kineo Matsui, A. Z. Tirkel, Fred Mintzer, B. Macq, Ahmed H. Tewfik, Frederic Jordan, Naohisa Komatsu, Joseph O'Ruanaidh, Neil Johnson, Ingemar Cox, Minerva Yeung, and Lawrence O'Gorman.
The artisan is assumed to be familiar with the foregoing prior art.
In the following disclosure it should be understood that references to watermarking encompass not only the assignee's watermarking technology, but can likewise be practiced with any other watermarking technology, such as those indicated above.
Digital watermarking can be applied to myriad forms of information. This disclosure focuses on its applications to security documents. However, it should be recognized that the principles discussed below can also be applied outside this area.
Most of the prior art in image watermarking has focused on pixelated imagery (e.g., bit-mapped images, JPEG/MPEG imagery, VGA/SVGA display devices, etc.). In most watermarking techniques, the luminance or color values of component pixels are slightly changed to effect subliminal encoding of binary data through the image. (This encoding can be done directly in the pixel domain, or after the signal has been processed and represented differently—e.g., as DCT or wavelet coefficients, or as compressed data, etc.)
While pixelated imagery is a relatively recent development, security documents—commonly employing line art—go back centuries. One familiar example is U.S. paper currency. On the one dollar banknote, for example, line art is used in several different ways. One is to form intricate webbing patterns (sometimes termed “guilloche patterns”) around the margin of the note (generally comprised of light lines on dark background). Another is to form gray scale imagery, such as the portrait of George Washington (generally comprised of dark lines on a light background).
There are two basic ways to simulate grey-scales in security document line art. One is to change the relative spacings of the lines to effect a lightening or darkening of an image region. FIG. 1A shows such an arrangement; area B looks darker than area A due to the closer spacings of the component lines. The other technique is to change the widths of the component lines—wider lines resulting in darker areas and narrower lines resulting in lighter areas. FIG. 1B shows such an arrangement. Again, area B looks darker than area A, this time due to the greater widths of the component lines. These techniques are often used together. Ultimately, a given region simply has more or less ink.
In my application Ser. No. 08/438,159 I introduced, and in my application Ser. No. 09/074,034 I elaborated on, techniques for watermarking line art by making slight changes to the widths, or positions, of the component lines. Such techniques are further expanded in the present disclosure.
In several of my cited applications, I discussed various “calibration signals” that can be used to facilitate the decoding of watermark data despite corruption of the encoded image, such as by scaling or rotation. Common counterfeiting techniques—e.g., color photocopying, or scanning/inkjet printing—often introduce such corruption, whether deliberately or accidentally. Accordingly, it is important that watermarks embedded in security documents be detectable notwithstanding such effects. Calibration signals particularly suited for use with security documents are detailed in this disclosure.
In some embodiments, security documents are encoded to convey machine-readable multi-bit binary information (e.g., digital watermarks), usually in a manner not alerting human viewers that such information is present. The documents can be provided with overt or subliminal calibration patterns. When a document incorporating such a pattern is scanned (e.g., by a photocopier), the pattern facilitates detection of the encoded information notwithstanding possible scaling or rotation of the scan data. The calibration pattern can serve as a carrier for the watermark information, or the watermark can be encoded independently. In one embodiment, the watermark and the calibration pattern are formed on the document by an intaglio process, with or without ink. A photocopier responsive to such markings can take predetermined action if reproduction of a security document is attempted. A passport processing station responsive to such markings can use the decoded binary data to access a database having information concerning the passport holder. Some such apparatuses detect both the watermark data and the presence of a visible structure characteristic of a security document (e.g., the seal of the issuing central bank).
One embodiment provides a method of generating a machine readable code for a printed document. The method generates a variable auxiliary message comprising message values and maps the message values to a two dimensional pattern of areas on the document. The areas each comprise an arrangement of pixels on the printed document. Depending on a value of the message mapped to a corresponding area, the method assigns a visible structure to the corresponding area. Different visible structures are assignable to a single value of the message to create a visible design having a desired aesthetic appearance.
One aspect of the invention is a printed security document. The security document comprising artwork including a security pattern, characterized in that the security pattern has the form of a line structure in which lines width or line spacing is adjusted to carry predefined data. The artwork is generated by the modifying at least one color of at least a part of the artwork to embed the security pattern in the artwork.
Another aspect of the invention is a method of generating artwork for a security document. This method provides a security pattern modulated to carry predefined data; and modifies width or line spacing of the artwork of the security document according to the security pattern. The artwork is generated by the modifying at least one color of at least a part of the artwork to embed the security pattern in the artwork.
Another aspect of the invention is a method of detecting a security pattern embedded in a security document. This method performs a frequency domain transform of artwork in the security document. The line width or spacing of line structures of the artwork is generated during document creation to incorporate the security pattern. The method detects the security pattern by correlating the transformed artwork with a predefined signal. The security pattern has frequency domain attributes that enable detection of the security pattern despite rotation of the security document.
Another aspect of the invention is a method of detecting a security pattern embedded in a security document. This method comprises analyzing artwork to detect the security pattern in modifications of the line width or spacing of line structures of the artwork; and performing plural statistical analyses on the detected security pattern to read predefined data carried in the security pattern.
The foregoing and other features and advantages of the present technology will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.